you have to do bench science. you have to look at the human body and what occurs in terms of changes in immune function, brain function. you can't just do epidemiology where you're comparing groups of children against each other. >> narrator: vaccine critics say many more studies are needed before scientists can rule out side effects like autism. studies of the other vaccines, studies of multiple additives, and studies to see if certain children are genetically predisposed to a vaccine reaction. >> the hypotheses continue to shift. the first hypothesis, which, you know, people bought into long and hard is that the combination measles, mumps, rubella, mmr vaccine cause autism. 12 epidemiological studies showed that that wasn't true. then the hypothesis shifted to thimerosal, an ethylmercury- containing preservative that was in vaccines, that's no longer in vaccines with... except for some multi-dose preparations of flu vaccine, that that caused autism. and that clearly has been shown not to be true. so now-- and th